Crude oil or petroleum is a complex mixture of hydrocarbons of varying molecular weights found in rock formations in the earth. The hydrocarbons present in the crude oil may be classified as aliphatic, alicyclic, aromatic or a mixture of such structures. The hydrocarbon compounds may contain oxygen, nitrogen, sulfur and traces of metals. Some of the lighter hydrocarbons are gaseous at atmospheric conditions. Besides hydrocarbon gases, carbon dioxide, hydrogen sulfide, and some inert gases may also be associated with crude oil.
The hydrocarbon compounds present in the crude oil may undergo many changes during their generation, migration and storage. Some changes are caused by interaction with bacteria and dissolved oxygen present in the flowing aquifer water under the hydrocarbon deposits. Such interaction oxidizes compounds present in the crude to alcohols, aldehydes and acids. Furthermore, the bacterial activity may also generate hydrogen sulfide and sulfur derivatives of hydrocarbons when the reaction conditions become anaerobic.
The compounds present in the crude oil may also be characterized as Saturates, Aromatics, Resins and Asphaltenes (SARA) based on their solubility characteristics in polar and non-polar solvents. Asphaltenes are defined as compounds present in crude oil which are insoluble in n-pentane. Aromatics and resins fractions are although soluble in n-pentane but easily get adsorbed on activated silica gel.
Saturates are the n-pentane soluble fraction which do not get adsorbed in activated silica gel. The difference between aromatics and resins fraction is defined by their elution characteristics from the activated silica gel. Aromatics get easily eluted by toluene from the activated silica get while resins are eluted by using some polar solvent like a mixture of methanol and toluene.
Such a classification of heavy compounds present in crude oil is quite arbitrary. From the chemistry point of view, Asphaltenes are heavy polar compounds having fused benzene ring structure with presence of some alkyl side chain moieties. They may also have some hetroatoms in the moiety such as oxygen, nitrogen and sulfur. The presence of metal ions such as nickel, molybdenum, vanadium, iron, magnesium etc. has been widely reported.
Aromatics are hydrocarbon compounds having 1-3 fused benzene rings with or without 1-2 alkyl side chains. Resins are generally organic acids having linear, alicyclic, aromatic or mixed structure. Saturates are alkanes having linear or branched chain structure. Linear alkanes of carbon number 14-40 are characterized as paraffin waxes and are also termed as macro crystalline waxes. When the carbon number increases from 40, these alkanes are termed as microcrystalline waxes. The maximum carbon number of saturates present in the crude is subject to the method of analysis and detection limit of the instrument but waxes having more than 100 carbon number have been reported.
Heavy compounds are sparingly soluble in crude oil. Asphaltenes are generally not soluble but remain in a colloidal state stabilized by polar compounds such as resins. Thus these heavy compounds remain dissolved or dispersed in crude oil at reservoir temperature and pressure. Such equilibrium is attained over geological times. Production from the reservoir disturbs this equilibrium bringing about separation (precipitation) of heavy compounds. The gases dissolved in crude oil which act as solvent for the heavy compounds start liberating from the crude oil as pressure is decreased. It affects the solubility of compounds like resins which play a vital role in stabilizing asphaltenes colloidal suspension, causing coagulation of asphaltenes. Such coagulated asphaltenes may agglomerate and deposit at rock/metal surface causing numerous problems.
Wax deposition in petroleum industry is a widely recognized problem. Wax deposition takes place as a result of a change of solubility equilibrium either due to a decrease in temperature or loss of light ends (gases dissolved in crude oil) at lower pressures. In such a situation, wax crystallizes out and tends to deposit at any surface like rock or metal. Heavy waxes (microcrystalline) precipitate first in order of their molecular weight.
Deposition is a dynamic process which is governed by diffusion of wax crystal (by Brownian motion) to substrate surface, cohesive forces between wax crystals and adhesive forces between wax crystals and substrate. Gravity settling of separated wax crystals may also contribute to some extent. The force of flow (shear force) opposes deposition and chips off deposited wax crystals.
Over a period of time, the nature of deposit changes and acquires a more thermodynamically stable form. Such stable form increases the melting point of deposits 10-15° C. more than the temperature of deposition environment. Thus any attempt to remove the deposit may not be successful if the temperature during treatment is not increased by 10-15° C. above deposition environment.
Rocks are initially water-wet i.e. their surface offers attraction to polar molecules like water and allow the easy movement of oil. Organic deposition particularly asphaltenes deposition on the rock surface changes the nature of such wettability to oil-wet. Once the rocks become oil-wet, they offer resistance to the flow of oil through its pore space. Thus it not only physically blocks rock pores (formation pores, formation pore throats) but also changes the wettability causing increased resistance to flow of oil thereby reducing productivity of oil wells.
The change in wettability increases the resistance to flow of oil, thus water is allowed to move more freely through rock spaces. This phenomenon is termed as change in relative permeability of water. When relative permeability of water in a rock formation increases with respect to oil and pressure differential across the rock faces is high, water coning may occur. Water coning may indicate a false water breakthrough.
Wettability of the rock can only be restored by removal of deposition and cleaning of the surface of deposited organic species.
Solid deposition may occur at borehole (wellbore formation), production tubing, flow lines or pipe lines. Deposition of organic solids. is very detrimental to production efficiency of the well if it happens in and around borehole (wellbore formation). This adverse effect on production is known as formation damage or development of skin or borehole damage.
An oil well is constructed by drilling a hole in the earth surface. After drilling each section of the well, a metallic pipe is lowered and cemented in the well. Such a pipe is called a casing. The last such casing lowered and cemented in the well serves the purpose of isolating the oil bearing rocks and is known as production casing. The oil flow in the casing is accomplished by perforating the casing at appropriate intervals. Sometimes the oil bearing rock is left uncased. Such a well construction is termed as bare foot completion. Another metallic pipe is lowered in the well which is called production tubing through which oil flows to the surface. The space between production casing and production tubing is called as annulus. Production tubing is sometimes isolated from the annulus by means of a packer. Sometimes another tube is also lowered inside the tubing (macaroni tube) to facilitate treatment of the producing fluid or other well interventions. Access to the annulus, production tubing and macaroni is provided by placement of suitable opening and valves contained at the surface in the Christmas tree.
The following methods may be applied to remove deposition of heavy organic compounds in the well bore, production tubing and production system.                1. Hot oiling, hot water circulation, steam squeeze or circulation—Temperature of deposition environment is raised to dissolve wax        2. Solvent squeeze—uses better solubility of solvents to waxes        3. Surfactant squeeze—uses dispersive property of surfactants        
The above methods may be effective to a certain extent. However, the repetitive use of these methods tends to diminish their effectiveness since the deposits get tougher and tougher as a result of the accumulation of higher molecular weight species.
Thermo-chemical formulations may involve the use of acids and bases or redox pairs. The present invention utilizes the heat generated from the exothermic reaction between acids and bases to remove organic deposits.
Inorganic acids like sulfuric acid, hydrochloric acid and bases like sodium hydroxide, potassium hydroxide have been used in aqueous solutions. Some times, such acids and bases have been used as suspension or emulsion in organic solvents.
U.S. Pat. No. 6,234,183 B1 discloses a method and composition for removing deposits of heavy hydro carbonaceous materials and finely divided inorganic particulate matter from wellbore and flowline surfaces using a composition containing an alkyl polyglycoside, an ethoxylated alcohol, a caustic and an alkyl alcohol.
United States Patent publication No. US 2004/0058827 A1 discloses paraffin inhibitors prepared by admixing a polymer having the characteristic of inhibiting paraffin crystalline growth in formation fluid from oil and gas wells with a first solvent selected from the weak to moderate wax solvents and a second solvent selected from the strong wax solvents. Examples of weak to moderate wax solvents include benzene, toluene, xylene, ethyl benzene, propyl benzene, trim ethyl benzene and mixtures thereof. Examples of strong wax solvents include cyclopentane, cyclohexane, carbon disulfide, decalin and mixtures thereof. The solvent system disclosed has desirably better solubility with polymers, even at reduced temperatures, than either solvent alone.
United States Patent publication No. U.S. Pat. No. 6,003,528, WIPO publication number WO 9831917, British Patent Publication number GB2307497 and Canadian Patent Publication number CA2115623 describe a thermo-chemical system for removal of wax deposits by generation of heat and nitrogen using two aqueous solutions of oxidizing and reducing nitrogen salts respectively. These aqueous solutions are emulsified into a non-polar solvent such as kerosene. Upon mixing the two emulsions, nitrogen and a mild amount of heat is generate. Redox reaction can be delayed by certain additives.
Russian Patent Publication RU 2203411 describes emulsion of aqueous solutions of ammonium nitrite and ammonium sulphite in organic solvent. The two emulsions generate heat upon mixing which helps the removal of wax deposits.
Russian Patent Publication number RU 2215866 describes the mixing of aqueous solutions of sulfamic acids and aqua ammonia to generate heat which helps the removal of organic deposits.
WIPO Publication number WO 2007060544 describes the mixing of aqueous solutions of sodium hydroxide and acetic acid to generate heat which is used for the removal of organic deposits.
Publication number MXPA 02007131 describes the esterification reaction between two solutions of polyhydric alcohol and carboxylic acid as a source of heat for the removal of organic deposits.
However, the abovementioned prior art does not provide a satisfactory solution as most of them involve the use of aqueous solutions, which are not very effective as components of organic deposits are not water soluble. Treatment with aqueous formulations presents emulsion and disposal problems.